Modern internal combustion engines in motor vehicles are equipped with systems for tank ventilation. These prevent highly volatile hydrocarbons situated in the fuel from passing into the environment.
For this purpose, the fuel tank is connected to a filter, preferably an activated carbon filter, which captures the hydrocarbons that escape from the fuel. The hydrocarbons that escape from the fuel accumulate in the filter. To ensure the functionality of the filter, it is necessary for the hydrocarbons that have accumulated there to be removed from the filter at regular intervals. This is performed in a purging process, in which the filter is purged with surroundings air. The filter is thereby regenerated.
The filter is connected via feed lines in each case to the fuel tank and to the surrounding atmosphere. The feed line from the fuel tank may preferably be equipped with at least one ventilation valve. The feed line from the surrounding atmosphere may preferably be equipped with a filter shut-off valve. The filter is connected via a further feed line to the intake pipe. A tank ventilation valve is situated in said feed line between the filter and intake pipe.
During the purging process, surroundings air is drawn from the surrounding atmosphere into the filter by means of negative pressure in the intake pipe with the tank ventilation valve open. In this way, the filter is purged with fresh air. The mixture of hydrocarbons and fresh air drawn out of the filter is supplied via the intake pipe to the internal combustion engine.
To comply with legal regulations, it must be possible to detect the functionality of the tank ventilation valve situated in the tank ventilation system through suitable diagnoses. In particular, it must be possible to identify a defective tank ventilation valve through suitable diagnoses.
Previous methods for diagnosis of the tank ventilation system are based on a diagnostic process performed additionally for this purpose, in which the functionality of the tank ventilation valve is inferred either from the resulting fuel-air mixture at the internal combustion engine or from the resulting pressure in the intake pipe.
For example, for diagnosis of the functionality from the resulting pressure in the intake pipe, in a first step, the tank ventilation valve is closed. In a second step, a settling phase is allowed to elapse. In a third step, the tank ventilation valve is opened with a targeted opening pattern. In a fourth step, with the aid of a pressure sensor, the pressure prevailing in the intake pipe and/or in the feed line between intake pipe and tank ventilation valve is determined. In a final evaluation step, a correlation factor is determined from the measured pressure and the applied opening pattern. With this factor, the functionality of the tank ventilation valve is monitored.
A disadvantage of the previous method is that, for carrying out the diagnosis, the operational tank ventilation must be interrupted. The interruption of the operational tank ventilation results in a reduction of the purging flow rate. Furthermore, in the event of an interruption of the diagnosis process, for example owing to changed operating conditions, the diagnosis process must be repeated, which further reduces the purging flow rate.